Exhaust gas sensors may be operated to provide indications of various exhaust gas constituents. For example, U.S. Pat. No. 5,145,566 describes detecting water content in the exhaust gas. Water content estimated using an exhaust gas oxygen sensor may be used to infer an ambient humidity during engine operation. In still other approaches, the water content may be used to infer a fuel alcohol content of a fuel burned in the engine.
However the inventors herein have identified potential issues with such an approach. Specifically, changes in exhaust air-fuel ratio can impact the output of the oxygen sensor, confounding the inferred results. Specifically, when the exhaust fuel ratio is richer than stoichiometry, the pumping current output by the oxygen sensor upon application of a voltage may be higher than expected. The error in pumping current results in an error in a corresponding humidity and fuel alcohol content estimation. Since humidity and fuel alcohol content are factors in determining engine operating parameters, such as injection amount, EGR amount, etc., errors in humidity and/or fuel alcohol content estimation can translate into degraded engine performance. In some approaches, the air-fuel ratio may be controlled to a target air-fuel ratio and the water content estimation may be performed only when the air-fuel ratio is at the target value. However, this not only relies on accurate air-fuel ratio control but also requires fuel adaptation to be completed before the water content can be estimated. As a result, during lengthy fuel adaptations, the water content estimation is delayed.
Thus, in one example, some of the above issues may be addressed by a method for an engine comprising, during a first engine fueling condition, applying a first voltage to an exhaust gas sensor, and learning an air-fuel ratio correction factor based on a sensor output. Further, during a second engine fueling condition following the first fueling condition, alternating between applying first and second voltages to the sensor, and estimating an injected fuel alcohol content based on sensor outputs at the first and second voltages and the learned correction factor.
Thus, in one example, the sensor outputs may be corrected to compensate for changes in air-fuel ratio. Specifically, responsive to application of the first and second voltages, first and second pumping currents may be generated. The first pumping current may be indicative of an amount of oxygen in a sample gas while the second pumping current may be indicative of the amount of oxygen in the sample gas plus an amount of oxygen contained in water molecules in the sample gas. The first and second pumping currents may then be corrected based on deviations of an expected air-fuel ratio (at which the engine is thought to be operating) from an estimated air-fuel ratio (at which the engine is actually operating). The corrected values may then be used to compute a water content, and infer an ambient humidity and an alcohol content of burned fuel with higher accuracy and reliability.
In this way, exhaust water content estimation and fuel alcohol content determination can be performed rapidly and accurately without requiring air-fuel ratio control. Specifically, the estimation can be performed without requiring the air-fuel ratio to be accurately controlled to a target value. In other words, the approach corrects for the air-fuel ratio being different from the target rather than controlling the air-fuel ratio to the target. As such, this allows water and alcohol content estimation to be performed without requiring fuel adaptation to be completed for accurate open-loop fueling. The inventors have also recognized that by using the same exhaust gas sensor to determine ambient humidity (for example, when the engine is operating without fueling, such as during a deceleration fuel shut-off), fuel alcohol content (for example, during a condition other than after a fuel tank re-fill), and an exhaust gas air-fuel ratio correction factor (for example, when the engine is operating fueled), component reduction benefits can be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.